Dysregulated tissue repair and inflammation characterise many common musculoskeletal pathologies1, including tendon disorders. Tendinopathies represent a common precipitant for musculoskeletal consultation in primary care2-3 and comprise 30-50% of all sports injuries3. Tendinopathy is characterised by altered collagen production from subtype 1 to 3 resulting in a decrease in tensile strength that can presage clinical tendon rupture4.
Inflammatory mediators are considered crucial to the onset and perpetuation of tendinopathy5. Expression of various cytokines has been demonstrated in inflammatory cell lineages and tenocytes suggesting that both infiltrating and resident populations participate in pathology6-9. Mechanical properties of healing tendons in IL-6-deficient mice are inferior compared with normal controls10 while TNF-α blockade improves the strength of tendon-bone healing in a rat tendon injury model11. While these data raise the intriguing possibility that cytokine targeting could offer therapeutic utility, there is currently insufficient mechanistic understanding of cytokine/matrix biology in tendon diseases to manifest this possibility in practice.
Interleukin 33 is a member of the IL-1 cytokine family that in turn plays a major role in innate immune responses. IL-33 is expressed in endothelial cells and fibroblasts, co-located with chromatin in the nucleus12. IL-33 is released following cellular damage13 and biomechanical overload14, and is thus considered an ‘alarmin’15. It has been implicated in a variety of inflammatory pathologies including pulmonary, cutaneous and articular diseases16. IL-33 functions via its cognate receptor ST2 that exists in membrane bound (mST2) or soluble form (sST2) and signals via a canonical IL-1R signaling cascade. Cytokines are often regulated at the post-transcriptional level by microRNA (miRNA) that control gene expression by translational suppression and destabilization of target mRNAs17. microRNA networks are emerging as key homeostatic regulators of tissue repair with fundamental roles proposed in stem cell biology, inflammation, hypoxia-response, and angiogenesis18.
MicroRNAs (miRs) are small non-coding RNAs that have a substantial impact on cellular function through repression of translation (either through inhibition of translation or induction of mRNA degradation). MicroRNAs derive from primary RNA transcripts (pri-miRNA) synthesised by RNA pol II, which may be several thousand nucleotides in length. A single pri-miRNA transcript may give rise to more than one active miRNA.
In the nucleus, the Type III RNAse enzyme Drosha processes the pri-miRNA transcript into a precursor miRNA (pre-miRNA) consisting of a stem-loop or hairpin structure, normally around 70 to 100 nucleotides in length. The pre-miRNA is then transported to the cytoplasm, where it is processed further by the RNAse Dicer, removing the loop and yielding a mature double stranded miRNA molecule, having an active “guide” strand (typically 15 to 25 nucleotides in length) hybridised to a wholly or partially complementary “passenger” strand.
The mature double stranded miRNA is then incorporated into the RNA-induced silencing complex, where the guide strand hybridises to a binding site in the target mRNA.
The guide strand may not be completely complementary to the target binding site. However, a region of the guide strand designated the “seed” sequence is usually fully complementary to the corresponding sequence of the target binding site. The seed sequence is typically 2 to 8 nucleotides in length and located at or near (within 1 or two nucleotides of) the 5′ end of the guide strand.
It is believed that single unpaired guide strands may also be capable of being incorporated into RISC. It is also believed that modifications to the passenger strand (e.g. to the sugars, the bases, or the backbone structure) which impede incorporation of the passenger strand into RISC may also increase efficiency of target inhibition by a double stranded miRNA.